In the present field of wireless communications, spectrum resources are mainly classified into two types: a licensed spectrum resource and an unlicensed spectrum resource. The licensed spectrum resource is a spectrum resource allocated by a radio regulatory commission of a government for a particular purpose, for example, a spectrum resource used by a mobile operator or a dedicated spectrum resource for civil aviation, railways, and police. Because of policy exclusivity, service quality of the licensed spectrum resource usually can be ensured, and it is relatively easy to schedule and control the licensed spectrum resource.
The unlicensed spectrum resource is also a spectrum resource allocated by a related government department. However, a radio technology, an operating enterprise, and a service life are not limited, and service quality of such a frequency band is not ensured either. A communications device can use the unlicensed resource for free, as long as requirements of indicators such as transmit power and out-of-band leakage are met. Common communications systems using the unlicensed resource for communication include a civil walkie-talkie, a radio remote control, a Wi-Fi system, a Bluetooth communications system, and the like. In the United States, unlicensed spectrum is defined by the Federal Communications Commission (FCC) under 47 CFR 15.
In an existing Long Term Evolution (LTE) system, a spectrum resource used by an operator is mainly the licensed spectrum resource. As a quantity of users on a mobile communications network increases, and user requirements on a communication rate and service quality become higher, an existing licensed spectrum resource can hardly meet a requirement of the operator's existing service. Considering that a new licensed spectrum is expensive and in shortage, the operator starts to pay attention to the unlicensed spectrum resource, hoping to use the unlicensed spectrum resource to offload a network capacity and improve service quality.
LAA-LTE (licensed-assisted access using LTE) is based on a configuration and a structure of CA (carrier aggregation) in the existing LTE system. That is, a base station uses a carrier on a licensed spectrum as a primary component carrier and a carrier on an unlicensed spectrum as a secondary component carrier. The network capacity is offloaded by using the unlicensed spectrum resource, thereby reducing load of the carrier on the licensed spectrum.
To ensure friendly coexistence of systems and devices that perform communication using the unlicensed spectrum, a listen before talk (LBT) channel access mechanism is introduced in some countries and regions, such as Europe and Japan. A basic idea of LBT is as follows: Before sending a signal on a channel, each communications device needs to detect whether the current channel is clear, that is, whether a nearby node is occupying the channel to send a signal. This detection procedure is referred to as clear channel assessment (CCA). If it is detected that the channel is clear within a period of time, the communications device can send a signal; or if it is detected that the channel is occupied, the communications device cannot send a signal at present. Specifically, in accordance with a provision of a European regulation, the LBT mechanism is further classified into an LBT mechanism for frame based equipment (FBE) and an LBT mechanism for load based equipment (LBE). A channel access mode of the Wi-Fi system is a kind of LBE, and is specifically a carrier sense multiple access with collision avoidance (CSMA/CA) mechanism. In a channel access mechanism of Wi-Fi, when a collision is detected in the system, a contention window grows exponentially, and a length of the contention window may reach 1024. To ensure channel access fairness, a relatively long contention window is likely to be set in the LAA-LTE system. Therefore, when the base station in the LAA-LTE system and multiple other devices using the unlicensed spectrum resource contend for an unlicensed spectrum channel, the LAA-LTE base station may take a relatively long period of time to access the unlicensed spectrum channel. In this case, detection always performed by the user equipment needs to consume a large amount of power.